In the recent energy-saving and resource-saving age, there have eagerly been demanded low cost, high performance carbon fibers used as a starting material of composite materials having a light weight, high strength and high modulus of elasticity as required for the production of airplanes and motorcars and molding carbon materials having a high strength and high density which can be compression-molded into various products. The present invention provides a process for producing an optically anisotropic carbonaceous pitch having a low softening point, high homogeneity and excellent molecular orientation which can be melt-spun into filaments and which are suitable for the production of the above carbon fibers and molding carbon materials of a high performance.
After intensive investigations on optically anisotropic pitch compositions suitable for the production of high performance carbo fibers as described in the specification of our prior Japanese Patent Application No. 162972/1980, the inventors have found that optically anisotropic pitches have well developed, condensed polycyclic aromatic laminate structure and a high molecular orientation, that in fact, there are various types of optically anisotropic pitches and that among those pitches, pitches having a low softening point and suitable for the production of homogeneous carbon fibers have a specific feature of chemical structures and composition. More particularly, the inventors have found that in the optically anisotropic pitches, compositions, structures and molecular weights of component O (i.e. n-heptane-soluble component) and component A (i.e. n-heptane-insoluble and benzene-soluble component) are quite important. More particularly, the inventors have found that a pitch composition containing specific contents of components O and A can be obtained as a perfect, optically anisotropic pitch and that it is an indispensable condition of an optically anisotropic pitch composition for the practical production of high performance carbon materials to suitably control the balance of the constituents.
It has further been found that if benzene-insoluble components (other than components O and A) in the pitch composition, i.e. benzene-insoluble and quinoline-soluble component (hereinafter referred to as component B) and benzene-insoluble and quinoline-insoluble component (hereinafter referred to as component C) are also specified, an optically anisotropic pitch for the production of a more excellent high performance carbon material can be obtained.
In addition, after investigation on relationships between (1) properties of the respective components and contents thereof and (2) physical properties, homogeneity and orientation of the whole pitch, the inventors have found also that the respective components must be contained therein in specific amounts and that the respective components must have specific properties.
There have been proposed several processes for the production of optically anisotropic carbonaceous pitches suitable for the production of high performance carbon fibers. However, an optically anisotropic carbonaceous pitch comprising components O, A, B and C suitable for the production of carbon materials having a high strength and high modulus of elasticity cannot be obtained by conventional processes. Further, the conventional processes have the following defects:
(1) Starting materials are not easily available on a large industrial scale. PA1 (2) A long reaction time or complicated steps are required and the process cost is high. PA1 (3) If the optically anisotropic phase content is increased to near 100%, softening point of the pitch is elevated to make the spinning difficult. On the other hand, if the softening point is lowered, the pitch becomes heterogeneous and the spinning thereof becomes difficult.
More particularly, in a process disclosed in the specification of Japanese Patent Publication No. 8634/1974, expensive starting materials such as chrysene, anthracene and tetrabenzophenazine which are not available in large amounts are used or complicated steps of dry distillation of tar obtained by cracking a crude oil at a high temperature followed by filtration of unmelted matter at a high temperature are necessitated and, in addition, a spinning temperature as high as 420.degree.-440.degree. C. is required. A process disclosed in the specification of Japanese Patent Laid-Open No. 118028/1975 relates to the conversion of a tar obtained by cracking a crude oil at a high temperature into a heavy product by heating under stirring. In this process, a long reaction time and the removal of unmelted matter are necessitated for obtaining a pitch having a low softening pitch. In the specification of Japanese Patent Publication No. 7533/1978, there is disclosed a process for polycondensing a petroleum tar or pitch in the presence of a Lewis acid catalyst such as aluminum chloride. However, this process is complicated and a high operation cost is required, since the removal of the catalyst and heat treatment before and after the removal are required. A process disclosed in the specification of Japanese Patent Laid-Open No. 89635/1975 comprises thermally polymerizing an optically isotropic pitch under reduced pressure or while an inert gas is introduced in the liquid phase till an optically anisotropic phase content of 40-90% has been attained. The specification of Japanese Patent Laid-Open No. 55625/1979 discloses an optically anisotropic carbonaceous pitch having an optical anisotropic phase content of essentially completely 100%. However, the pitch has considerably high softening point and spinning temperature. The starting materials used in this process are limited vaguely to some commercially available petroleum pitches. If starting materials such as coal tar and petroleum distillation residue are used for the production of pitch in this process, the resulting pitch has an excessive molecular weight. As a result, unmelted matter is formed and the softening point and spinning temperature are elevated to make the spinning difficult. Thus, in the previously proposed processes for the production of optically anisotropic carbonaceous pitch, the composition or structure of the starting materials are not specified and, therefore, according to those processes, it is impossible to stably provide a carbonaceous pitch of a constant high quality.